Drawing on the Canvas - HTML5 Canvas - page 42

Under the hood, a mathematical matrix operation applies to all transformations. Luck-
ily, you do not need to understand this to use simple Canvas transformations. We will
discuss how to apply rotation, translation, and scale transformations by changing sim-
ple Canvas properties.
Rotation and Translation Transformations
An object on the canvas is said to be at the 0 angle rotation when it is facing to the left
(this is important if an object has a facing; otherwise, we will use this as a guide).
Consequently, if we draw an equilateral box (all four sides are the same length), it
doesn't have an initial facing other than one of the flat sides facing to the left. Let's
draw that box for reference:
//now draw a red square
context.fillStyle = "red";
context.fillRect(100,100,50,50);
Now, if we want to rotate the entire canvas 45 degrees, we need to do a couple simple
steps. First, we always set the current Canvas transformation to the “identity” (or “re-
set”) matrix:
context.setTransform(1,0,0,1,0,0);
Because Canvas uses radians, not degrees, to specify its transformations, we need to
convert our 45-degree angle into radians:
var angleInRadians = 45 * Math.PI / 180;
context.rotate(angleInRadians);
Lesson 1: Transformations are applied to shapes and paths drawn after the setTransform() or other
transformation function is called
If you use this code verbatim, you will see a funny result… nothing ! This is because the
setTransform() function call only affects shapes drawn to the canvas after it is applied.
We drew our square first, then set the transformation properties. This resulted in no
change (or transform) to the drawn square. Example 2-7 gives the code in the correct
order to produce the expected result, as illustrated in Figure 2-12 .
Example 2-7. Simple rotation transformation
function drawScreen() {
//now draw a red square
context.setTransform(1,0,0,1,0,0);
var angleInRadians = 45 * Math.PI / 180;
context.rotate(angleInRadians);
context.fillStyle = "red";
context.fillRect(100,100,50,50);
}